The present invention relates to an impregnating oil as a dielectric medium for a capacitor, prepared by admixing 0.1 to 10 parts by weight of an isocyanate-compound of tolylene diisocyanate and/or 4,4-diisocyanate-3,3-dimethyldiphenyl with 100 parts by weight of an insulating oil of 1-tolyl-2-phenylethane, 1-ethylphenyl-2-phenylethane, 1-isopropylphenyl-2-phenylethane and/or 1,2-ditolylethane, and an oil impregnated capacitor comprising at least two plastic films repectively coated with a vacuum-evaporated metal on one surface thereof (hereinafter referred to as the metallized plastic film capacitor) and impregnated with the thus prepared impregnating oil.
The capacitor having an element prepared by piling at least two films respectively coated on one of the surfaces thereof with a vacuum-evaporated metal (such a film is hereinafter referred to as the metallized film) such that the metallized surface of one of the two metallized films is contacted with the non-metallized surface of the other metallized film, winding the thus piled films and impregnating with an insulating oil shows an excellent self-healing properties for the dielectric breakdown and has a merit of the possibility of being available in small size and light weight. However, on the other hand, a demerit of capacitance decrease of such a type of capacitors during use under high voltage owing to the partial oxidation of the metal from the metallized surface of the metallized film has been recognized. Further, in case of applying higher dielectric stress on the capacitor, the capacitance decrease is easily apt to occur, and such a capacitance decrease is more easily caused in the case where the vaccum-evaporated metal is, for instance aluminium.
Several reasons are considered as the cause of the capacitance decrease in the capacitors, for example, (1) the presence of the partial oxidation of the metal of the metallized film caused by the presence of moisture both in the element of the capacitor and in the insulating oil, (2) the presence of phenolic antioxidants contained in polypropylene film and (3) the presence of a soldering paste containing rosin used for welding a lead wire to the sprayed metal of the capacitor element. Accordingly, in order to prevent the capacitance decrease in such a type of capacitors, particularly metallized polypropylene film capacitors (hereinafter referred to as MPP capacitors), methods for removing moisture from the insulating oil and/or the element thereof have been studied. However, because of the difficulty of completely removing the moisture industrially and experimentally, it has been impossible to sufficiently prevent the capacitance decrease of MPP capacitors.
Further, although trials have been carried out by thickening the metal layer on the metallized film for preventing the capacitance decrease of MPP capacitors, since the self-healing property of MPP capacitors becomes poorer with the thickening of the metal layer and as a result, the specific merit of the capacitor provided with an element comprising the metallized films impregnated with the impregnating oil is spoiled, such a method cannot provide any capacitors having an element comprising the metallized films impregnated with the impregnating oil of a practical value.
Furthermore, although the capacitance decrease of MPP capacitors is improved to a certain extent by substituting zinc for aluminum as a vacuum-evaporated metal, since the zinc layer of the metallized film is inferior in the oxidation stability in air to the aluminum metallized film, the zinc metallized film has a demerit of necessitating troublesome treating thereof.
As a result of the studies of the inventors of the present invention, it is found that the addition of the isocyanate compound of tolylene diisocyanate and/or 4,4-diisocyanate-3,3-dimethyldiphenyl to the insulating oil of 1-tolyl-2-phenylethane, 1-ethylphenyl-2-phenylethane 1-isopropylphenyl-2-phenylethane and/or 1,2-ditolylethane affects the moisture content of the capacitor element, and the present invention has been attained.